An electrosurgical generator is used in surgical procedures to deliver electrical power to the tissue of a patient. An electrosurgical generator includes a radio frequency generator and its controls. When an electrode is connected to the generator, the electrode can be used for cutting or coagulating the tissue of a patient with the high frequency electrical energy. During operation, current flows from the generator through an active electrode, through the tissue and bodily fluids of a patient, and back to the generator through a return electrode. The electrical circuit formed by the electrodes and the patient is referred to as the patient circuit.
The electrical energy has its waveform shaped to enhance its ability to cut or coagulate tissue. Different waveforms correspond to different modes of operation of the generator, and each mode gives the surgeon various operating advantages. Modes may include cut, coagulate, a blend thereof desiccate or spray.
One problem that may be encountered in the use of electrosurgical equipment is that the electrode will arc to the patient as the electrode is withdrawn from the tissue. This is due to power control systems in the electrosurgical generator which are designed to increase the output voltage as higher impedances are presented by the tissue. This is usually done to maintain the output power. In the case of withdrawal of the electrode, the control system in the electrosurgical generator may function as though the tissue impedance has increased dramatically and try to maintain power delivery. The control system may rapidly increase the voltage, thus causing the active electrode to arc to the tissue as it is withdrawn, This phenomenon is called "exit sparking," and it is undesirable because it causes unwanted tissue damage. Designers of electrosurgical generators want to minimize this outcome.
Exit sparking may also occur because electrical energy is stored in the capacitive and inductive elements of the final amplifier stage in an electrosurgical generator. Even though the generator output can be turned off once the electrode is removed from the tissue, the energy that is stored in the final amplifier stage must be dissipated. If no other dissipation path is available, the stored energy might arc to the patient.
Early designs for electrosurgical generators avoided the problems of exit sparking is several ways. One way to avoid exit sparking is to prevent high voltages from occurring at the active electrode. High voltages can be prevented by actively controlling the output voltage or else by passive means. In many early generators, the electrical capabilities of the generator where not sufficient to produce high voltages at the active electrode when the impedance of the load was high, and therefore the problem of exit sparking was passively avoided.
Closed loop power control systems in modern electrosurgical generators may enhance the possibility of exit sparking. Closed loop control of output power could cause the output voltage of the generator to rise as the impedance of the tissue is increased. As the active electrode is withdrawn from the tissue, the measured impedance of the load can rise sharply. The closed loop control system increases the voltage in response to this perceived rise in impedance. The resulting high output voltage could cause an exit spark.
U.S. Pat. No. 4,969,885 discloses an active control apparatus for controlling the output voltage of an electrosurgical generator. Since exit sparking is most likely a problem in generators that control output power (rather than output voltage), the '885 patent does not contemplate the problem of exit sparking.
U.S. Pat. No. 5,099,840 discloses an electrosurgical generator that adjusts the resonant frequency of its output stage in accordance with the impedance of the load. This is done to increase the efficiency of the output stage. The problem of exit sparking is not contemplated in the '840 patent. In contrast to the '840 patent, the present invention seeks to decrease the gain, and hence decrease the efficiency of the output stage in order to avoid exit sparking. This is the opposite result of the '840 patent.
Other U.S. Patents have related technology, but none are directed at the problem of exit spark control. U.S. Pat. No. 4,658,819 has a circuit wherein the power delivered to the electrode is a function of the voltage from a DC supply and the load as measured by sensors of load voltage and current. A microprocessor controller digitizes the sensing signals and computes the load impedance and actual power being delivered. The microprocessor controller accordingly repeats the measurement, calculation and correction process approximately as long as the generator is operating. U.S. Pat. No. 4,372,315 discloses a circuit which measures impedances after delivering a set number of radio frequency pulses on a pulse burst by pulse burst basis. U.S. Pat. No. 4,321,926 has a feedback system to control electrosurgical effect delivery but the impedance sensing is not on a real time basis. U.S. Pat. Nos. 3,964,487, 3,980,085, 4,188,927, and 4,092,986 have circuitry to reduce the output current in accordance with increasing load impedance. In those patents voltage output is maintained constant while the current is decreased with increasing load impedance. U.S. Pat. No. 4,094,320 has a circuit that responds to impedance changes as measured by sensing current in the active and return leads. The sensed currents are subtracted from one another and if that exceeds a variable threshold the generator is turned off. The variable threshold is a function of power level and leakage current through stray capacitance.
One of the purposes of the present invention is to overcome the problem of exit sparking while still allowing for high power at the active electrode.